1. Field of the Invention
This invention relates to computer systems, and more particularly, to methods and apparatus for regulating the output voltage of charge pumps used to generate high voltage levels from lower level source voltages.
1. History of the Prior Art
There has been a recent trend toward lowering the power requirements of portable computers. In order to reduce power consumption, much of the integrated circuitry used in personal computers is being redesigned to run at low voltage levels. The circuitry and components used in portable computers are being designed to operate at voltage levels such as 5 volts and 3.3 volts. This helps a great deal to reduce the power needs of such computers.
However, some of the features of portable computers require voltages higher than these supply voltages. For example, flash electrically-erasable programmable read only memory (flash EEPROM memory) has begun to be used to store the basic input/output startup (BIOS) processes used by a computer. This memory may be erased and reprogrammed without removing the BIOS circuitry from the computer by running a small update program when the BIOS processes are to be changed. However, erasing and reprogramming flash EEPROM memory requires approximately twelve volts to accomplish.
Another type of flash EEPROM memory array provides another example of high voltage requirements in portable computers. This type of flash EEPROM memory array provides a new form of long term random access storage. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is described in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. Such an array provides a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is important. These flash EEPROM memory arrays also require much higher voltages and substantially more power for programming and erasing data than can be provided directly by the batteries of low powered portable computers.
One way in which the power requirements of the flash EEPROM arrays have been reduced is through the use of negative gate erase techniques. These techniques reduce the amount of current required during the erase process and thereby reduce the power used.
In other electronic arrangement, charge pump circuits have been used to provide a high voltage from a lower voltage source. However, it is necessary to provide very accurate voltages for programming and erasing the cells of a flash EEPROM memory array. Charge pumps and other circuitry implemented as a part of integrated circuits provide output voltages which typically vary over a range of source voltages, operating temperatures, process variations, and load currents. Moreover, when using charge pumps the voltage level provided at an output terminal tends to vary substantially from a desired value. This occurs because the output of a charge pump is furnished in a series of charge pulses which are stored to provide an output voltage. The charge pump generates this form of output because the output and other stages function in the manner of a diode which only transfer charge in a properly biased condition. This creates a voltage ripple on the output of the pump when the current requirements are high relative to the capacitance of the load.
These problems make it desirable to attempt to regulate the output voltage provided by the charge pumps to obtain a relatively constant value which is not affected by variations in source voltage, operating temperature, process, or load current. Moreover, it is especially desirable to regulate the output voltage provided by charge pumps utilized to generate voltages used for negative gate erase operations.